Quote
sungod3k
I ordered a 20mhz atmega 1284p replacement for my sanguinololu
These 20 MHz aren't a fixed frequency, but a maximum frequency. ATmegas run at lower frequencies just fine, down to the kHz level.
@majic79 is right with clock source selection, but fuses for 16 MHz and 20 MHz are the same. On what to set, see [reprap.org]